Thyrite protective device



G. L. USSELMAN THYRITE PROTECTIVE DEVICE May 2% 19 Filed Oct. :2, 1936 2 Sheets-Sheet 1 H. F. OSCILLATOR INVENTOR e. L. USSELMAN W65 AT loRNEY y 1949- G. L. 'ussaLmAw THYRITE PROTECTIVE DEVICE Filed Oct. 2, 1956 2 Sheets-Sheet 2 N. A m w m TS N m W w G/ Y B Patented May 21, 1940 UNITED STATES PATENT OFFICE 2,201,211 THYRITE PROTECTIVE DEVICE Application October 2, 1936, Serial No. 103,643

2 Claims.

This invention relates to an improved protective device and more particularly to a circuit protective device for the protection of apparatus subjected to high voltage surges, such as filament bypass condensers and filament transformers generally employed in a transmitting circuit.

Filament bypass condensers and transformers have heretofore been manufactured with a sufficiently high safety factor above its normal working voltage so that it was not generally necessary to employ any protective device to protect the condensers and transformers against occasional increased voltages. However, with the advent of mercury vapor tube rectifiers' now employed in the higher powered transmitters, trouble with condenser and transformer failure has greatly increased due to higher voltage surges. Several attempts have been made to protect the condensers and transformers, such as to shunt spark gaps in rare gas across the condensers for protection, but this method has not proven to be reliable.

In the present invention, the filament bypass condensers and filament transformers are protected by novel circuits employing a particular form of material, sold under the name Thyrite," which is described more fully in United States Pat. #1322342, to McEachron, of September 8, 1931, and the General Electric Review for February, 1930, pages 92 to (vol. 33, No. 2).

The use of this Thyrite protective device permits the use of lower voltage bypass condensers, and consequently larger capacity can be obtained in the same size condenser case.

The present invention will best be understood by referring to the accompanying drawings, in which:

Fig. 1 is a wiring diagram of a transmitter circuit wherein the bypass condensers are protected by Thyrite discs;

Fig. 2 is an elevation partly in section of the Thyrite protective device;

Fig. 3 is an end view of Fig. 2;

Fig. 4 is another embodiment of a Thyrite protective device; while Fig. 5 is an end view of Fig. 4.

Referring now in detail to Fig. 1 of the drawings, an abbreviated transmitting circuit is shown which employs a protective device I, which is made up of alternateplates or discs 2 of Thyri and metallic electrodes 3, 4 and 5, the electrodes 3 and 4 each being attached to a terminal of filament bypass condensers 6 and l which are to be protected. The other electrodes 5 and 5' are connected to ground. From the circuit, it will be noted that the Thyrite is in parallel with the apparatus to be protected. In this case, the center points of the condensers 6 and l are connected to ground. The two outside terminals of the condensers are connected to the filament leads of the tube 8. The purpose of the bypass condensers 6 and I is to act as a radio frequency bypass to prevent the radio frequency from entering and damaging the filament transformer 9. It is also necessary that the filament leads be grounded for radio frequency currents for the proper operation of a transmitter.

During the operation of high powered transmitters employing mercury vapor tubes there frequently occurs an arc-over, due to various causes, across the neutralizing condenser H1 or in the tube 8, itself. In either case, the result is an instantaneous discharge of the smoothing condenser H in a high voltage rectifier l2. This discharge enters the filament of tube 8 and when protective devices are not used the current of the discharge must flow through the secondary Winding of the filament transformer 9, through the impedance l3 to ground. The impedance 13 may be such apparatus as overload relays, meters, resistors, etc.

In any case, the impedance I3 is usually high enough to cause abnormal voltages to occur from the terminals of condensers 6 and l to ground. It is not practical for certain reasons, such as space requirement and high manufacturing cost, to make the voltage rating of the condensers high enough to withstand more than about onetenth of the full rectifier voltage. However, in practical operation, condensers B and I are subject to several times their voltage rating when an arc-over, as mentioned above, occurs. This results in frequent condenser and filament transformer failures which continue to occur unless proper means of protection is used. A modulated source of high frequency oscillations 14 is connected to the grid coupling condenser 15 and ground 16.

Thyrite has the property of decreasing in resistance with an increase of current through it, which follows the law given more fully in the February, 1930, issue of General Electric Review, and the McEachron Pat. #1,822,742, of current=KV where V is the voltage across the Thyrite material, and K is some constant depending upon the size and type of the Thyrite plate. When the voltage is doubled, the current through the Thyrite plate increases about twelve times so that it will be seen that this material is ideal for a condenser protective device, and with proper mounting will protect the bypass condensers and filament transformers from dangerous voltages which might cause insulation failures of the transmitter, and as a consequence interruption of service.

Referring now to Figs. 2 and 3 of the drawings, a simple circuit protective device is shown which employs four Thyrite discs 2| which are assembled on an insulating rod 30 and clamped between two insulating end plates 22 of suitable insulating material. Two springs 23 are arranged each side of the discs 22 to press the "'Ihyrite discs and electrodes together. The electrodes 24, 25, 26, 21 and 28 are sandwiched in between the Thyrite discs 2| and in case of electrodes 21 and 28 between discs 2| and end plates 22. It is, of course, to be understood that the number of Thyrite discs may be increased in even number up to twelve. or even more discs, to increase the current rating of the protective device.

The discs 2| and insulating members 22 and the electrodes or terminals 23 to 28 inclusive are all provided with a central aperture through which passes the insulating rod 30. At each end of insulating rod 30 there is provided mounting brackets 3| and 32, red 30 being secured to the mounting brackets by suitable screws 33 and 34. To electrically connect the protective device to the condenser, there are provided mounting brackets 3| and 32 which are then secured to members 35 and 36. The outside members 25 and 26 and center terminal 24 are joined together by means of a clamping strip 24 which is electrically connected by means of flexible lead 39 to a ground terminal 39 on the frame of the transmitter. The electrode members 21 and 28 are connected to terminal members 38 and 35 by means of nuts on the two screws 32 and 3|,respectively. The terminal members 36 and 35 are connected to the terminals of the condensers 6 and I, as shown in Fig. 1. The midpoint of the condensers is connected to the condenser case and thence to ground.

The protective device shown in Figs.'4 and 5 is particularly adapted to transmitting circuits wherein a larger current carrying device is desired. The general arrangement is similar to that shown in Figs. 1 to 3 inclusive, except that the overall dimensions of the Thyrite discs and their number employed are increased. The Thyrite discs 40 are interposed between a pair of copper cooling fins 4| and are spaced apart from the adjacent group by a pair of spacing discs 42, preferably of aluminum. Members 44 to 42 inclusive are provided with a suitable aperture or hole through which an insulated rod or sleeve 43 passes. At each end of the protective unit there is located a cup-shaped disc-like spring element 44. A rod 45 which is threaded at each end passes through sleeve 43 and the entire unit is tightly clamped together by means of nuts 44, and a pair of end mounting members 41 is located at the end of nuts 48 and is securely fastened to the protective device by means of nuts 48.

Holes 49 and 50 are provided in plates 4|, as shown in Fig. 5. for electrical connection to the Thyrite protective device. With this arrangement, most any desired connection, such as series or parallel, may be obtained.

What is claimed is:

1. An electrical protective device comprising an alternate assembly of annular "Thyrite discs and annular metallic electrodes, each electrode having a projecting lug, an insulating washer at each. end of said assembly, an insulating rod passing through said assembly, binding means for maintaining said assembly in intimate contact including an L shaped metallic support at each end of said rod, and a spring member between each of said supports and said insulating washers, terminal lug extensions on each of said L shaped members and an insulating supporting strip between said members, the central and end electrodes of said assembly being connected together and connected to ground, connections from each of the intermediate electrodes to a corresponding one of said terminal lug extensions, said lug extensions being adapted to be connnected to an electrical circuit to be protected.

2. An electrical protective device comprising an assembly of annular Thyrite discs and annular metallic electrodes, each electrode having a projecting lug, an insulating washer at each end of said assembly, an,insulating rod passing through said assembly, binding means for maintaining said assembly in intimate contact including an L shaped metallic support at each end of said rod and a spring member between each of said supports and said insulating washers, terminal lug extensions on each of said L shaped members and an insulating supporting strip between said members, the central and end electrodes of said assembly being connected together and connections from each of the intermediate electrodes to a corresponding one of said terminal lug extensions.

GEORGE LINDLEY USSELMAN. 

